1. Field of the Invention
The present invention relates to a communication apparatus for a vehicle having a function of locating a portable unit in an electronic key system, and more particularly, it relates to such a vehicular communication apparatus suitable when applied to a passive entry technique to operate the locking and unlocking of the doors of a vehicle only with the portable unit in a simple and easy manner.
2. Description of the Related Art
In the past, as a communication apparatus for remotely controlling equipment mounted on a motor vehicle, it has been proposed a vehicular communication apparatus that has a function to automatically unlock the doors of the vehicle when a user carrying a portable unit is approaching the vehicle and to automatically lock the vehicle doors when the user is going away from the vehicle.
In such a known apparatus, however, a certain measure is taken not to lock the doors in an automatic fashion so as to prevent the portable unit from being confined to the vehicle when the portable unit is present therein. As a result, when the user is away from the vehicle with the portable unit left behind in the vehicle, the doors will remain unlocked, thus giving rise to a problem from the viewpoint of crime prevention.
Accordingly, in case where the user is going away from the vehicle with the portable unit being left therein, it is necessary to inform the user of such a state. In other words, a function is required to make a determination as to whether the portable unit exists inside or outside the vehicle.
As a technique to determine whether the portable unit is present in the vehicle or not (hereinafter, referred to as a “vehicle inside and outside determination technique”), there has been proposed a communication apparatus for a vehicle that uses an LF band radio wave of a frequency of about 120 KHz-135 KHz level, for instance (see, for example, a first patent document: Japanese patent application laid-open No. 2003-221954).
According to such a vehicle inside and outside determination technique using the LF band radio wave, antennas to radiate an LF band radio wave (hereinafter, referred to as an “LF band antenna”) are placed in various locations of the vehicle and connected to a on-board unit through wires or cables so that electric power and a predetermined frequency are supplied to the LF band antenna, respectively.
In addition, it is designed so that the portable unit is responsive to a radio wave radiated from the LF band antennas when the portable unit exists in a place in which the radio wave has a predetermined magnetic field strength or above, and the on-board unit makes a determination as to the existence of the portable unit based on a response from the portable unit to a designated LF band antenna.
On the other hand, an RFID (Radio Frequency IDentification) technique is known as a technique to detect an object by using radio waves (see, for instance, a non-patent first document: “RFID Handbook, second edition, Principle and Application of Contactless IC Card (ISBN 4-526-05291-4)” (written by Klaus Finkenzeller, translated by Soft Engineering Laboratory, published by Nikkan kogyo Shinbun Co.)).
As techniques for RFID, there are a lot of ones that are called by a variety of names such as a wireless IC tag, an IC tag, an RF tag, an ID tag, and so on.
The RFID system typically comprises an RF tag and a reader writer, and when a radio wave is radiated from the reader writer to the RF tag with both of them being present within a communicable range, an ID signal is sent from the RF tag so that the reader writer receives the ID signal and is able to obtain the existence of the RF tag and information according to the ID signal.
In the known communication apparatus for a vehicle, since LF band antennas for remote control are connected to the on-board unit through wires or cables, the arrangement or routing of wiring is needed, thus posing a problem that a large amount of labor is required for installation.
Particularly, since a determination as to whether the portable unit is present inside the vehicle is made by the use of LF band waves, there is a problem that the arrangement or routing of wiring to the LF band antennas becomes complicated, and the size of the LF band antennas is large, thus limiting mounting places thereof.
Moreover, since an LF band antenna is generally comprised of a coil with a conductor wrapped around a core that is composed of ferrite, etc., there are the following problems. That is, the reduction in size thereof is difficult, and the locations of installation of LF band antennas are limited according to the size of each antenna. In addition, there are constraints on the condition of arrangement and designing of antennas that are able to fulfill a satisfactory communication function without impairing the design of the vehicle.
Further, there is also the following problem. That is, though a plurality of antennas are needed so as to cover the entire area of the passenger compartment of the vehicle by using LF band radio waves, the cost of each single antenna unit is high because of the above-mentioned construction of each LF band antenna, and it is difficult to do cost reduction thereof, so the entire cost is increased due to an increase in the number of antennas used.